Allow a unix: address to contain a C-style quoted string.

Feature 18753 -- all this to allow spaces.

src/common/util.c

@@ -2906,7 +2906,7 @@ read_file_to_str, (const char *filename, int flags, struct stat *stat_out))
  * provided), and return a pointer to the position in <b>s</b> immediately
  * after the string.  On failure, return NULL.
  */
-static const char *
+const char *
 unescape_string(const char *s, char **result, size_t *size_out)
 {
   const char *cp;

src/common/util.h

@@ -385,6 +385,7 @@ MOCK_DECL_ATTR(char *, read_file_to_str,
 char *read_file_to_str_until_eof(int fd, size_t max_bytes_to_read,
                                  size_t *sz_out)
   ATTR_MALLOC;
+const char *unescape_string(const char *s, char **result, size_t *size_out);
 const char *parse_config_line_from_str_verbose(const char *line,
                                        char **key_out, char **value_out,
                                        const char **err_out);